Ink jet printing is a non-impact process in which an electronic signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers are broadly utilized as output for personal and other computers due to their reliability, print quality, low cost, relatively quiet operation and their graphics capability. Thermal or bubble jet drop-on-demand printers have been especially successful and are widely used in office, industrial and home sites.
A major concern with all ink jet printing devices is pluggage of nozzles during operation and between operations. A critical property for an ink jet ink is the decap time which is the length of time over which an ink remains fluid in a pen opening when exposed to air and therefore capable of firing a drop of ink at its intended target. Failure is often due to evaporation of solvent causing precipitation or crystallization of a solid at the air/liquid interface. In pigmented inks a further cause of pluggage is the flocculation of dispersed pigment due to a change in solvent composition caused by evaporation.
As evaporation proceeds an initial increase in viscosity may affect ink performance. As solid material begins to form the projected ink drops may be deflected from their intended trajectory or even worse may completely plug nozzles thereby causing total print failure. In a multinozzle printer, certain nozzles are assigned to infrequently used characters and these are the most likely to undergo pluggage failure. Also a series of nozzles must be able to start printing after a period of non-use.
A further complicating factor is the use of heating devices to accelerate ink drying on the paper. This also accelerates solvent evaporation in the nozzle and makes pluggage more likely.
A second critical ink property is the ability to penetrate paper or film rapidly so as to allow for rapid drying. One major deficiency of thermal ink jet printers is their relatively slow printing speed compared to printers using competing technologies. A major obstacle is the ink drying rate. In a sheet fed printer, the ink must be dry before the succeeding sheet contacts it or the ink will smear. This is the major factor affecting printer productivity. Rapid drying by penetration is an alternate to using a heater to increase drying. Use of a heater increases the purchase price of the printer.
The use of penetrants or cosolvents is known in the art as a means to increase the rate of penetration of inks. However, the known penetrants, such as Butyl Carbitol.RTM., have unacceptable or marginally acceptable decap times and cause pigment dispersions to flocculate.
A third important requirement for inks where the colorant is a pigment is for the pigment dispersion to remain stable throughout the life of the ink jet cartridge. Dye-based ink jet inks suffer from deficiencies in water fastness, smear fastness and lightfastness. Therefore pigments are a preferred alternative to dyes provided the pigment dispersions can be made stable to flocculation and settling. Many commonly used cosolvents cause destabilization of pigment dispersions and cannot be used either as penetrants or pluggage inhibitors.
These three requirements, pluggage inhibition, rapid media penetration and compatibility with pigment dispersions are often in conflict. Therefore it is difficult to obtain all three useful properties in a single substance. Rapid penetrants are mediocre or poor pluggage inhibitors. Pluggage inhibitors make poor penetrants. Both pluggage inhibitors and rapid penetrants are usually destabilizing toward pigment dispersions. Accordingly, a need exists for cosolvents that decrease drying time and increase decap time without destabilizing pigment-based inks.
This invention allows pigmented inks to have rapid dry time and long decap times without compromising pigment dispersion stability.